Calcium Isotope Holds Clue To Neutrino Mass Mystery

The secret to determining the mass of neutrinos, which are thought to be both matter and antimatter, could finally be unlocked by experiments measuring the decay of calcium-48.

AsianScientist (Apr. 22, 2016) – The negligible mass of neutrinos—subatomic particles that could be matter and antimatter at the same time—mystifies scientists around the world. Now, researchers in Japan have used one of the world’s most powerful supercomputers to analyze a special decay of the calcium-48 isotope, the life of which depends on the unknown mass of neutrinos.

This advance, published in Physical Review Letters, will hopefully facilitate the detection of this rare decay in underground laboratories.

Neutrinos were discovered more than 60 years ago. However, scientists have yet to learn some of their fundamental properties, such as their mass (for which only the upper limit is known; this is around 3.6 x 1036 kg), or whether neutrinos and antineutrinos are in fact the same particle.

An experiment that may offer an answer to the first of these questions is the so-called ‘neutrino-less double beta decay.’ This occurs when an atom’s parent nucleus decays into a daughter nucleus—gaining two protons, losing two neutrons and emitting two electrons.

One example of this is the decay of calcium-48, a very rare isotope of calcium with 20 protons and 28 neutrons, into titanium-48. This is the process that has now been analyzed and modeled in unprecedented detail by scientists from the University of Tokyo.

“The half-life of this decay depends on two factors: the unknown mass of neutrinos (which are part of the process, even though none are emitted) and the characteristics of the parent and daughter nuclei,” said study co-author, researcher Dr. Javier Menéndez.

“This implies that, knowing these nuclear characteristics, and once this decay has been measured experimentally in one of the underground laboratories working on it, it will be possible to determine the mass of neutrinos.”

The team’s achievement has been understanding the nuclear part “in a reliable way” through extremely complex quantum mechanics calculations. These operations were run using the world’s fourth fastest supercomputer, the K computer at the RIKEN Institute in Kobe, Japan.

“Our findings will make it possible to directly obtain neutrino mass when the half-life of this decay is measured experimentally,” says Menéndez.

“Moreover, they suggest that the decay of calcium-48 is around half as long as what was previously thought (2 x 1025 years, rather than 4 x 1025 years). This improves our chances of observing it.”

In any case, this is an extremely rare and slow decay as it is mediated by two simultaneous weak decay processes. This means that it takes trillions of years to occur and is very difficult to detect. Laboratories working on this subject hope to observe one (which is due to decay very soon) in deep underground mines, far from any external disturbances.

After presenting their findings with calcium-48, the researchers are now working on similar calculations for the neutrino-less double beta decay of germanium-76, selenium-82 and even xenon-136.

“The most interesting thing would be to confirm that neutrinos are not emitted during double-beta decay, as that would imply by physical principles that neutrinos and antineutrinos are the same particle; that would be a massive discovery,” stressed Menéndez.

“If that happened, we could say that neutrinos are Majorana particles, because they would be particle and antiparticle at the same time. This property was proposed by the Italian physicist Ettore Majorana in the 30’s.”

If neutrinos and antineutrinos are discovered to be the same particle, this would be the first known case of matter that is simultaneously antimatter. Additionally, it would generate an asymmetry that would serve to explain why there is no antimatter in the universe. Majorana neutrinos would have allowed for the creation of more matter than antimatter in the first moments after the Big Bang—for example, in neutrino-less double-beta decay, two electrons are emitted, the creation of matter, but no antineutrinos. After that, all antimatter would have been annihilated along with the majority of matter, releasing energy and leaving behind only the ‘excess’ matter, which can be observed in the universe today.

The article can be found at: Iwata et al. (2016) Large-Scale Shell-Model Analysis of the Neutrinoless ββ Decay of 48Ca.


Source: Spanish Foundation for Science and Technology.
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